The present invention relates to a system for controlling the transmission ratio of a transmission and the opening degree of a throttle valve for a motor vehicle.
Generally, a control system for a constant speed drive (hereinafter called a cruise control system) for a motor vehicle having a continuously variable belt-drive automatic transmission (CVT) is separate from a transmission ratio control system. Namely, the cruise control system operates to independently control a throttle valve of an engine regardless of the transmission ratio, so that the constant speed drive may be held. However, if load on the engine of the vehicle greatly changes, the constant speed drive cannot be maintained. For example, when the vehicle goes up a hill, the throttle valve is opened for increasing the engine power. When the throttle opening degree approaches a full-throttle position, the engine power is no longer increased. Accordingly, the vehicle speed becomes lower than the desired vehicle speed for the constant speed drive, since the transmission ratio is not controlled for the cruise control.
Japanese Patent Application Laid-Open No. 60-135335 discloses a cruise control system for a motor vehicle with a CVT wherein a current control means is provided for controlling current passing to a vacuum operated throttle actuator in accordance with the difference between a set vehicle speed Vs and an actual vehicle speed V. Correcting means is provided for correcting transmission ratio i for the transmission in accordance with current or a control signal for the current control means, thereby controlling the actual vehicle speed V to the set vehicle speed Vs.
In the conventional system, the transmission ratio i is corrected in accordance with an electric signal supplied to the vacuum operated actuator. However, the control operation of the actual vehicle speed is performed with a slight delay of time after the generation of the signal to the actuator. Accordingly, the engine power can not be properly controlled upon a large change of the engine load.